Precision resistance device



y 1952 A. G. ROBBINS 2,597,674

PRECISION RESISTANCE DEVICE Filed Oct. 29, 1949 Inventor": IAIexander-GQobbms,

b @4 WOLF.

' His Attorney.

Patented May 20, 1952 PRECISION RESISTANCE DEVICE Alexander G. Robbins,Schenectady, N. Y., assignor to General Electric Company, a corporationof New York Application October 29, 1949, Serial No. 124,337

4 Claims,

This invention relates to resistance devices, and more particularly toprecision resistance devices such as precision potentiometers andprecision resistors and methods of manufacturing the same.

One conventional form of precision resistance device comprises a basesuch as a cylinder of predetermined diameter and length, an electricalconductor of uniform cross-section throughout its entire length spirallywound on the base, contact means such as an electrical terminalconnected to one end of the conductor, and a movable contact meansarranged to slidably engage successive turns of the conductor as theformer is moved from one position to another.

In such a conventional form of precision resistance device the ratio ofmaximum to minimum resistance gradient, that is, the ratio of maximumchange in resistance for a given change in position of the movablecontact means to the minimum change in resistance for a comparablechange in position of the movable contact means. is attained by spacingsuccessive turns of the wound conductor. Those skilled in the art willunderstand that a precision resistance device as above described cannotbe wound to produce a high ratio of resistance gradient. To increase, byconventional means, the ratio of resistance gradient in a conventionalwire-wound resistance device using wire of uniform cross-section, thebase may be specially shaped. For example, a frusto-conically shapedbase may be used in place of a cylinder. By this means a ratio ofmaximum to minimum resistance gradient of approximately 10 to 1 isobtainable. Also, further increases in such ratio may be accomplished byemploying a wire of tapered cross-section in addition to a speciallyshaped base. However, although such expedients increase the ratio ofresistance gradient to some extent, the resistance 7 gradient is stillcontrolled by the spacing of the successive turns of the winding, andthis characteristic limits theextent to which the ratio of resistancegradient may be increased. This characteristic also prevents theattainment of a zero resistance gradient, that is, no increase inresistance for a change in the position of the movable contact means.

Moreover, in such a conventionally wound resistance device resistancechanges accompany the post-winding operations thereon. That is, afterthe wire is wound on the base, it is secured thereto by a bondingcement, and, due tothe pressing of the turns of wire during the curingof the bonding cement, the turns of wire are moved, thereby changing theresistance characteristics of the resistance device. Also, during thebonding operation, heat and pressure are applied to the turns of wire,thereby changing the stress in the wire which in turn changes theresistivity thereof.

It is, therefore, an object of this invention to provide a precisionresistance device in which the ratio of maximum to minimum resistancegradient is appreciably increased as compared to conventionally woundresistance devices having wire of uniform cross-section.

It is a further object to provide such a precision resistance device asabove described without the employment of tapered wire or a special- Ingeneral, my invention comprises in a pre-' cision resistance device abase which may be of any suitable shape, uniform in cross-section orotherwise, an electric conductive coating fixed to a surface of thebase, a first and second plu-' rality of slots in the conductivecoating, the slots being cooperatively positioned to establish aconductive path of a desired resistance characteristic through theconductive coating, and relatively movable contact means engaging theconductive coating to vary the resistance of the precision resistancedevice.

For a complete understanding of my invention reference should be had tothe following specification and to the accompanying drawing, in

which Fig. l is a diagrammatical view'of a pre-" cision resistancedevice illustrative of my invention; Fig. 2 is a diagrammatical view ofa precision resistance device illustrative'of my'invention and used todescribe a method of manufacture thereof; and Fig. 3 is a diagrammaticalview of a precision resistance device illustrative of my invention andused to describe a second method of manufacture thereof. It should benoted that each of the figures of the drawing for clarity areconsiderably enlarged as compared to the actual device and are not drawnto scale.

Referring to the drawing, in Fig.- 1 I have shown for the purpose ofillustrating my invention a precision resistance device I comprising abase 2 made of glass or other suitable insulation material and having anupper flat surface 3 to which an electric conductive coating 4 isapplied and fixed thereto, and a pair of relatively movable contactmeans 5 and 6. The conductivecoating 4 may be titanium, tungsten,

zirconium or any other suitable conductive material and may be appliedto the surface 3 by any suitable means, as for example, by any oftheconventional processes of vapor deposition, cathode sputtering, chemicalreduction, electroplating or the Schoop process. Contact means 5 isillustrated in the drawing as a fixed electrical terminal connected tothe base 2 adjacent an end I of device I and engaging the conductivecoating 4 to provide electrical connection between the contact means andconductive coating. Contact means 6 is arranged for slidable engagementwith conductive coating 4 adjacent an edge 8 thereof. Device I may beconnected to an electrical circuit through a pair of conductor means 9and ID respectively connected to contact means 5 and 6.

To'increase the resistance of device I and to provide desired finiteresistance values of the conductive coating4 between the contact means 5and 6, a first andsecond plurality of slots num bered II-I5 and IB-I9respectively are cut or otherwise formed in the conductive coating 4.The slots II'I9 may be of any desired shape and may perforate theconductive'coating '4 or extend partially therethrough, as desired.Slots II-I5 emanate from an edge 20 of the conductive coating 4 andextend in the direction of the opposite edge 8 thereof. Slots I6--I9 areinterspaced each between adjacent of theslots I II 5, 3t

as shown in the drawing, and emanate from edge8 of conductive coating 4and extend in the direction of edge 20 thereof. Thusa winding 'or zigzagpath of conductivity as indicated by the arrows 2| isestablished betweenthe relatively movable contact means 5 and 6.

By narrowing the path of conductivity 2I, and by lengthening the same,the resistance between contact means 5 and 6 is increased. Thusbyadjusting the length of the slots IB-I9 and the as I8, and arelatively short slot, 'such as IS, the

ratio of resistance gradient of the device I is appreciably increasedover the maximum-ratio of resistance gradient attainable in aconventional wire-wound potentiometer having wire of 'uniformcross-section. That is, assuming the distance between points 22 and 23and the distance between points Hand 25 to be equal, the ratio of changein the resistance of the device I, resulting from a movement of thecontact means 6 from point 22 to 23 to the change in resistanceresulting from moving the contact means 6 from point 24 to point 25, isgreater than can be provided in a conventional wire-Wound resistancedevice having wire of uniform cross-section. Therefore, by my inventionI provide a simple means for attaining a desired'high ratio ofresistance gradient without the necessity of a specially shaped base andtapered wire.

A'further advantage of my resistance device lies in its ability toprovide-any resistance gradient down to substantially a zero resistancegradient, that is, no appreciable change in resistance for a givenchange in the position of the movable contact means. For example,consider the points 25 and 21 and a movement of the contact means 6 frompoint 26 to point 21. In such a movement of contact means 6 noappreciable resistance change will be introduced because there are noslots in the conductive coating 4 between points 28 and 21 and a broadpath of conductivity is presented. Therefore, my resistance device can.bemade to produce more minute resistance changes for a given motion ofthe movable :contact,means and thereby provide more accurate adjustmentsthan conventional wirewound resistance devices.

Another advantage of my precision resistance device is that havingformed the necessary slots in the conductive coating 4 the devicerequires no further treatment except for a protective coating ofsynthetic resin or other suitable material, which coating produces .nodetrimental change in the resistance of the device. Thus, theundesirable resistance changes which accompany the post-Windingoperations on wirewound resistance'devices are eliminated and theaccuracy of my device is retained.

Another advantage of my device is that a conductive coating may beapplied to any shape of base, as for example,-to cylinders or toroids,and appropriately'slotted to producethe same result.

As a modification of my invention, to increase the number of resistancechanges for a given movement of the movable contact mean 6 and therebyprovide a still more accuratelyadjustable resistance device, the contactmeanst may be arranged slidably to engage the conductive coating '4along-a path indicatedby-the arrows 28 and the slots III9 may-bearrangedeach to cross the path 28 as exemplified by slots-.12 and I1. That is,considering the points -29 and 30 the distance between which is equal tothe distance between points 25 and 22, if the contact means 6 is movedalong'the path'28 between points 29 and. there isaresistance .changelasthe movable contact 6 passes over each of the slots 12 and I1; whereasfor an equivalent movement of the contact 6 along the edge 8 of theconductive coating 4 between the points 25 and 22 only one resistancechange takes place becauseonly one slot, namely II,.is crossed. Thus. byarranging the oppositely disposed first and second plurality of slotsII-I5 and I6--I9 in overlapping positions, each slot crossing the path28, as exemplified by slots I2 and I'L'and by arranging for the contactmeans '6 to follow a path crossing each slot the number of resistancesteps in the total resistance of the device 'I is doubled and theaccuracy of adjustment is thereby measurably increased.

To illustrate a first method of manufacture and further advantages of myprecision resistance device, attention is drawn to Fig. 2 wherein Fig. lis reproduced with appropriate additional references to describethe'above-mentioned first method of manufacture. To manufacturesprecision resistance device of my invention, as for example, device I,the base '2, which may be of any shape, is coated with aconductivematerial to form an area of conductive coating 4ha-vingopposite edges 8 and 20. Considering the contact termina15 and the point25 on the conductive coating 4 and neglecting all of theslots II-I9, ifcurrent is passed between terminal 5 and point 25 through the conductivecoating 4, it may follow various paths as indicated by the arrows 3| andthe resistance between the terminal 5 and point 25 is some finite valuedepending upon the thickness of the conductive coating 4 and the area ofthe conductive path indicated by the arrows 3|. Assume next that it isdesired to increase the resistance between terminal 5 and point 25 to apredetermined finite value, this may be accomplished by providing theslot [6 of predetermined length emanating from edge 8 of the conductivecoating 4 in the direction of the edge 20 thereof and therebyinterrupting a portion of the conductive paths 3!. Next assume that itis desired to establish to finite resistance between terminal 5 andpoint 22. This may be accomplished by providing the slot I! ofpredetermined length emanating from the edge 8 of conductive coating 4and extending in the direction of the edge 20 thereof. However, althoughthe slot [1 establishes a desired finite resistance between terminal 5and point 22, it also destroys the previously established finiteresistance between the terminal 5 and point 25 because slot llinterrupts certain of the conductive paths 3| not interrupted by theslot [6. 7

To avoid this undesirable result and thereby provide means forestablishing a finite resistance between terminal 5 and point 22 withoutdestroying a previously established finite resistance between terminal 5and point 25, in accordance with my invention the first plurality ofspaced slots lll5 of predetermined length emanating from the edge 25 ofthe conductive coating 4 and extending in the direction of the edge 8thereof, are

provided before the slots and I"! are made,

16 and then slot l7. By forming slot 16 a desired finite resistance isestablished between terminal 5 and point 25. Thereafter, by forming slotI! a finite resistance is established between terminal 5 and point 22.However, the forming of the slot I! does not change the desired finiteresistance between terminal 5 and point 25 because slot l1 interrupts noconductive paths between terminal 5 and point 25. The paths 3! whichmight have been interrupted by forming the slot [1 have previously beeninterrupted by the slots H and I2 which slots were formed before thefinite resistance between terminal 5 and point 25 was established by theslot IS. The same reasoning holds for finite resistances establishedbetween terminal 5 and successive points on the conductive coating 4.Therefore, by first forming the plurality of slots Hl5 and thereaftersuccessively forming slots along the edge 8 of conductive coating 4, aplurality of finite resistance values of the device I may be establishedfor each position of the movable contact means 6, and the establishmentof each succeeding resistance will have no effect on a previouslyestablished finite resistance.

To determine the length of each of the slots l6-|9, the device I iselectrically connected in a Wheatstone bridge or other suitableelectrical measuring circuit and each slot is out or otherwise formedfrom the edge 8 of the coating 4 in the direction of the edge 20 thereofuntil the correct resistance value is attained. For example, to form theslot l5, terminal 5 and point are electrically connected to a suitableelectrical resistance measuring circuit (not shown) and a cutting tool(notshown) is applied to the conductive coating 4 at the edge 8 thereofand moved toward the edge 20 thereof until the measuring circuitindicates that the resistance between terminal 5 and point 25 is adesired value. Thereafter, the measuring circuit is electricallyconnected between terminal5 and point 22 and'the" process repeated toform the slot [1. Additional slots are formed along the edge 8 ofconductive steps as previously described the slots emanating from edge 8of conductive coating 4 and the slots emanating from edge 20 thereof maybe arranged to overlap and the movable contact means 6 arranged tofollow a path crossing the slots emanating from both edges 8 and 20 ofthe conductive coating 4.

However, in the manufacture of a precision resistance device of theconductive coating type, it may be desirable to provide additionalthickness to the portion of the conductive coating which is slidablyengaged by the movable contact means to allow for attenuation thereof.Such portion shall hereinafter be referred to as the contact path. Infollowing the first method of manufacture above described, suchadditional thickness of the contact path must be cut by the cuttingtool. This is undesirable because a cutting tool comprising a cuttingmember, such as a diamond, may readily be arranged to cut the thinconductive coating, but when applied to the thicker contact path, maytear the conductive material adjacent to the desired cut.

Therefore, to avoid cutting through the contact path, a second method ofmanufacture employing photo-engraving may conveniently be applied. Forexample, referring to Fig. 3 in which I have shown a precisionresistance device 32 of the conductive coating type illustrative of myinvention, such precision device may be conveniently manufactured in thefollowing manner. First, a special glue 33 such, for example, as topping:enamel, Which i soluble in cold water if unexposed to light andotherwise soluble only in warm water, is coated on a base 34 ofinsulation material such as glass. Base 34, for simplicity ofdescription is shown rectangular in shape with the understanding thatany suitable shape of base may be employed, as for example, acylindrical or disk-shaped base. Thereafter, the special glue 33 isexposed to a light pattern of desired shape by any suitable conventionalmeans (not shown) such as a light source and screening means to exposeto light that portion of the coating of glue 33 defined by the shadedareas 35. The base 34 is then washed with cold water which removes allof the glue 33 from the base with the exception of the portion adheringto the shaded areas 35. The latter portions are not removed by the coldwater washing because the exposed glue is not soluble in cold water. Aconductive coating 36 is thereafter applied to the base 34 by anysuitable process such as cathode sputtering, vapor depositing,electroplating, the Schoop process, or chemical reduction and whenapplied adheres strongly to the base only where there is no glue. Theconductive coating 36 is thereafter masked by any conventional meanssuch as masking paint to leave a contact path indicated by numeral 31unmasked. An additional thickness of conductive coating is thereafterapplied to the contact path, by any of the above-mentioned processes,

the masking paint or other material is removed,

and the base is washed in warm water to remove the exposed glue from theshaded areas 35 thereby also removing the conductive coating from thearea 35. The result of the operations above described is the formationof a blank 38 comprising the-base 34, the-conductive coating 36,thecontact' path 31 of additional thickness of conductive coating and afirst and second plurality of slots 38-41 and 42--44 emanatingrespectively from opposite edges 45 and 4B of the conductive coating 35through the contact-path 31, the slots 39-44 being formed when theexposed glue is washed from the base 34 by warm water.

To form a precision resistance device ofpredetermined resistancecharacteristics such as device 32 from the blank 38 a stationary contactmeans 41 is connected to the base 34 adjacent an end 48 thereof toengage the conductive coating of the contact path 3'! and a movablecontact means is arranged to slidably engage the contact path 3?. Toestablish a finite resistance between the contact means 41 and 49 forvarious positions of the contact means 49 along the contact path 31 itis necessary only to cut or otherwise form an additional length to theslots 39 in successive order from the terminal 41. That is, a finiteresistance between contact means 41 and each of a plurality of points5055 as indicated in Fig. 3 along the contact path 31 is established bycutting or otherwise forming an additional length to each of the slots39, 42, 40, 43, 4i and 44 successively in the order named. For example,to establish a desired finite resistance between contact means 4! andpoint 59, slot 39 is extended asufficient distance as indicated by thedotted portion5$ to provide the desired finite re- Thereafter. a desiredfinite resistance ween contact means 41 and point 5| is established inthe same manner by extending the slot 42 a indicated by the dottedportion 51 a suillcient distance to provide the second mentioned desiredfinite resistance. The establishment of each successive finiteresistance will effect no change in previously established finiteresistance value for reasons previously mentioned with respect to Fig.2. In order to align the cutting tool. or other means of forming theadditional length to each slot, the relative position of the cuttingtool (not shown) and the blank 33 is controlled by photo-electric orother suitable means (not shown) using the slots 39-44 in the blank 38as a register.

This method of forming a precision resistance device is particularlyadvantageous in that the method is readily adaptable to high speedmanufacturing technique and is reliable in producing accurate results.

Moreover, by this method the disadvantages previously described withreference to cutting through the heavy thickness of. conductive materialof. the contact path 3-! are avoided since no cutting of the material ofthe contact path is required;

While I have shown and described particular embodiments of my invention,it will be obvious to those skilled in the art that various changes andmodifications may be made without departing from my invention in itsbroader aspects and I, therefore, aim in the appended claims to coverall such changes and modifications as fall within the true spirit andscope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

An electrical potentiometer comprising a baseof clectiinsulationmaterial, a coating of electrical resistance material on saidbase, a sec- 0nd coating of resistance material extending: from one endof said base tov the otherandcov ering a narrow portion of the. centralarea of said first coating to define a brush path of double thickness ofconductive coating, said coatings including transverse slots alternatelyextending inwardly from opposite side edges of said device completelyacross said second coating but not completely across said first coatingto provide a zig zag resistance path having a predetermined desiredresistance characteristic.

2. A precision potentiometer resistor comprising a, strip of electricalinsulation. material. a coating of electrical conductive materialextending from one end to the other of said strip in a narrow band alongthe central section of one face thereof to define a brush path, a secondcoating of conductive material extending across the entire area of saidface, said conductive coatings including slots alternately extendinginwardly from opposite edges of said strip, each slot extending entirelyacross said narrow coating strip and partially through said secondcoating.

3. A precision potentiometer resistor device comprising an insulatingbase, a, path for a movable contact extending along the central portionthereof, a coating of electrically conductive ma.- terial on the surfaceof said base including said contact path, said coating including anextra thickness of conductive material in the area of said contact path,said coating also including transverse slots alternately extendinginwardly from the opposite edges of said base, each slot extending atleast through the area of said contact path.

4. A method of manufacturing a precision resistor potentiometer of theslotted conductive coating type having a contact path of greaterresistance material thickness than the remainder of. the potentiometercomprising the steps of applying a photosensitive adhesive to aninsulating base, applying a pattern of light to said photosensitiveadhesive corresponding to the pattern of the resistor desired, applyinga first uniform coating of conductive resistance material, applying acoating of masking material over the entire surface not included in thecontact path, applying a second uniform coating of conductive materialon said contact path and then removing. the portions of thephotosensitive adhesive as determined by the light pattern together withthe portions of the first and second coatings of conductive materialsupon the portions of the insulating base covered by the adhesive whichis removed.

ALEXANDER G. ROBBINS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,061,107 Schellenger Nov. 17,1936 2,159,106 Richter May 23, 1939 2,273,941 Dorn i- Feb. 24.. 1942FOREIGN PATENTS Number Country Date 406,634 Great Britain May 23, 1932461,275 Great Britain Feb. 1'5, 193'?

